For counting, we use thousand, million, billion, trillion etc. For physical quantities, thousand becomes Kilo, million is Mega and (short scale) billion is Giga. In both these systems, the consecutive prefixes differ by an order of $10^3$. For currency, the first set of prefixes is used but for physical quantities, the second set is used. Why use two different systems?
This is speculation, but some advantages of the SI system are:
It's much easier to distinguish "kilo" from "milli" than "thousand" from "thousandth", and easier to distinuguish "mega" from "micro" than "million" from "millionth".
In writing, "kg" is much quicker to write than "thousand grams". We could maybe still make an abbreviated system based on "thousand" and "million" but we'd need a way to distinguish "thousand" from "thousandth" and so on, probably resulting in somewhat arbitrary abbreviations like "k" and "$\mu$" being used. Then we'd likely naturally make up names for these abbreviations for when we read them aloud. And we might end up with something very like what we have.
"kg" means the same thing in every language, but "thousnd grams" would be written differently in English, German, French, Chinese, etc. So the system aids international communication between scientists.
Further, if you consider Chinese and Japanese (and probably other languages I know less about), they don't traditionally divide their numbers every 3 decades but every 4. In Chinese, 10,000 isn't "ten thousand" but its own number, "wan". 100,000 is not "one hundred thousand" but "ten wan". Using SI gives a regular system across cultures.